Mississippi River mystery: No young cottonwoods

An unheralded icon of the banks of the Mississippi River -- cottonwood trees -- might be disappearing in the metro.

Despite the ubiquitous spring snowstorms of seeds loosed by mature trees, young cottonwoods have all but disappeared from the Mississippi National River and Recreation Area that stretches 72 miles through the metro.

Researchers don't know why the trees are failing to become adults, but they suspect unnaturally stable water levels, created by the lock-and-dam system, might be partly to blame.

In a cruel twist of fate, the river's current flooding is thwarting researchers' hopes to plant young trees in St. Paul and Minneapolis as part of a research-and-restoration project.

"They need flooding, but not right now," said Maria DeLaundreau, an ecologist working with the Mississippi River Fund that is heading the project.

Cottonwoods in general aren't in trouble. Fast-growing saplings are sprouting throughout their range as well as in the metro, often to the consternation of prairie restoration project leaders.

But those trees are in upland areas. Along the banks of the Mississippi, as well as the Minnesota River and probably some rivers out west, young trees have all but vanished.

In 2012, the National Park Service, which operates the Mississippi National River and Recreation Area, surveyed the woods in the river's floodplain. They found no trees smaller than 15 cm in diameter.

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Big ones were aplenty, but no next generation has appeared for decades.

"Everyone I've talked to has been completely shocked when they learn that, and no one I've talked to had any idea before then," said DeLaundreau, who works for Minnesota GreenCorps and is assigned to the Mississippi River Fund. The fund is a nonprofit that partners with the National Park Service to benefit the national river and rec area.

Cottonwoods are "essential" to the ecosystem along the river's fluctuating edge. A "pioneer" species that quickly populates an area, they rapidly stabilize soil and draw moisture, allowing other forest plants to thrive. As towering mature trees, which have shapes reminiscent of American elms, they shade the water, moderating temperatures for fish, and provide crucial habitats for bald eagles. In one study, 84 percent of bald-eagle nests in the national park were in cottonwoods.

The Achilles' heel of the cottonwood is perhaps its seeds. While prolific, the cotton-wisp-carried seeds are only viable for a matter of weeks. (Seeds from some Minnesota trees remain viable more than a century later.)

"They need to come into contact with a place to grow pretty quickly," DeLaundreau said. "They need bare, wet soil and full sun -- the kind of conditions you see after a flood."

Cottonwood seeds can't grow in water, and while adult trees can withstand prolonged flooding, seedlings cannot. "So they need to drop their seeds after the flooding," she said.

The locks and dams along the river aid commercial shipping, in part by moderating the natural fluctuations in water levels -- essentially avoiding low-water periods -- and DeLaundreau said she and others suspect the lack of scoured sandbars and other once-common river features is making it difficult for cottonwoods to grow.

Climate change might be playing a role, too. The river's current flooding, for example, is late in the typical flood season and the result of extreme rainfalls that have become increasingly common in recent years.

The goals of the Mississippi River Fund's project, which is in its first year, are several.

One is to ensure the trees remain part of the river's ecology by planting a bunch of them.

But first, DeLaundreau said, forest ecologists need to learn basic things about cottonwoods, such as: What are the best ways to plant them?

"It's such a basic question, but cottonwood trees are not known for their high-quality timber, so they really haven't been studied much," she said. "That's the most interesting part of this to me: how little attention cottonwoods have received, especially considering their ecological value."